Physical Review Applied

Superconducting qubits must be initialized in their ground state with very high fidelity, for quantum computing and sensing. Conventional reset schemes are limited, as they operate the qubit within the same noisy electromagnetic environment used for its everyday control. This study couples a transmon qubit to a high-overtone bulk acoustic resonator, a physically distinct bath that is intrinsically colder than its electromagnetic surroundings. The authors use its multimode structure to repeatedly extract entropy from the qubit. This simple, feedback-free protocol yields residual excited-state populations one to two orders of magnitude lower than for typical schemes.

Transition-edge sensors (TESs) have already been proven to detect electrons with kinetic energy of about 100 eV, with a Gaussian energy resolution of 1 eV, comparable to the photon energy resolution of the same device. This study investigates how changes in the experimental setup influence the energy resolution of TES devices, for electrons produced by a ‘cold’ source of vertically aligned carbon nanotubes. Decreasing the size of both TES and electron source, the energy resolution for electrons significantly improves by a factor of more than 21. These results open up possibilities for the high-resolution spectroscopy of low-energy electrons, for e.g. the measurement of neutrino mass.

Light emission via inelastic electron tunneling (LEIT) is an ultrabroadband light source with potential impact in visible-light communication, intelligent optical sensing, and on-chip optoelectronics. Its low efficiency is typically addressed using plasmonic tunneling junctions, but their subwavelength size results in omnidirectional radiation with poor collimation. The authors combine a plasmonic tunneling junction with a metasurface to collimate LEIT to a narrow divergence angle across a broad spectral window. The supported hybrid plasmon-photon modes both enhance the local density of states and extend spatial coherence, mitigating the trade-off between response speed and collimation.

Magnon-phonon hybridization enables coupled control of spin and mechanical excitations, but the limited frequency tunability of conventional surface-acoustic-wave (SAW) devices is restrictive. The authors develop a SAW platform with a fundamental frequency of 193 MHz, enabling quasicontinuous mapping of magnon-phonon resonances up to 5.6 GHz in an epitaxial Co${}_2$FeSi film. Magnon-induced SAW absorption is enhanced near the transition between monostable and bistable magnetization states; even so, the two regimes exhibit distinct frequency scalings. This approach provides a route toward tunable, potentially energy-efficient magnonic devices and dynamic spin control in hybrid systems.

What happens when a photon is absorbed by a superconductor? Although vortex-antivortex pairs have long been suspected to play a central role in superconducting photon detection, their dynamics following photon absorption have eluded direct observation. By monitoring quantized voltage signals generated by photon-induced vortices, the authors reveal their generation statistics and demonstrate photon-number resolution. This work provides a rare microscopic view of photon detection in superconductors, and opens a route toward fast superconducting photon-number-resolving detectors.

Atomic interferometers offer extraordinary inertial sensitivity, yet their intrinsically periodic response has long prevented continuous operation over a wide dynamic range. The authors demonstrate a dual-channel closed-loop atomic beam interferometer, enabling simultaneous quantum feedback for rotation and acceleration. By converting periodic matter-wave interference into continuously tracked control parameters, the scheme overcomes the conventional half-fringe limitation while preserving precision. This advance brings practical quantum inertial navigation a significant step closer.

Millimeter waves sit at the energy scale of many collective excitations in quantum materials, but probing microscopic samples at these frequencies is difficult: Spectroscopic alignment is hard in a cryostat, and superconducting cavities stop working in high magnetic fields. This Letter reports an all-silicon (no metal or superconductor) photonic crystal cavity functioning as a chip-scale conductivity sensor near 100 GHz, reaching a quality factor above ${10}^5$ at 4.3 K. The all-dielectric platform should work at the strong fields and low temperatures where quantum Hall edge modes, magnetoplasmons, and field-tuned correlated phases exist, and it may be scalable to terahertz frequencies.

This work introduces a unified platform supporting topological states for both electromagnetic and elastic waves, within the same artificial crystal. While topological photonic and phononic systems typically are investigated independently, the approach here enables direct comparison of their topological properties and transport behaviors in the selfsame geometry. Breaking a specific spatial symmetry in a graphenelike lattice opens valley-polarized topological band gaps in both physical domains, yielding interfacial states between topologically distinct crystals. The authors establish a general, unifying framework for topological photonics and phononics.

Starting from a quartz-crystal microbalance, the authors develop a battery-free wireless sensing technology that enables remote measurements of structural strain and gas concentration over distances exceeding 50 m. The system enhances the electromechanical coupling between an AT-cut quartz resonator (packaged in a slightly pre-bent state) and electromagnetic waves, enabling long-range sensing without onboard power sources or electrical connections. The ability to perform battery-free long-range sensing is promising for smart infrastructure monitoring, with applications in bridges, pipelines, nuclear facilities, and industrial plants.

Stopping and localizing elastic waves can concentrate energy for high-sensitivity sensing and harvesting, but this often requires intricately tuned lattices or symmetry-restricted piezoelectric transducers. This study finds that flexoelectricity—the universal coupling between strain gradients and electric polarization—could be combined with higher-order elasticity to create stable, tunable zero-group-velocity extrema and stationary-inflection modes. Introducing a defect cavity for tighter localization increases both open-circuit voltage and mass responsivity. The resulting self-sensing, electrically reconfigurable resonators could enable compact devices for diverse applications.

Fast, high-fidelity qubit measurement and initialization are important for minimizing errors in quantum algorithms. Latched readout is a powerful technique for high-fidelity measurement of a spin qubit, but with it come inherently long initialization times. This Letter presents a fresh initialization technique that takes advantage of a fast, two-step decay process to achieve better than 50-fold speedup over passive initialization of latched readout states. This multistep technique uses a low-amplitude pulsing scheme to achieve fast qubit initialization, comparable to simpler single-step techniques, while relieving technical complications that can arise.

Powerful and efficient numerical techniques have been central to theoretical research on quantum mechanical systems for decades. In the era of quantum advantage demonstrations, it is paramount to develop strong benchmarks that truly represent the classical frontier. This study presents a high-performance parallel implementation and large-scale demonstration of quantum dynamics simulated with OR-represented quantum algebra at a huge scale, retaining over a trillion Pauli strings while maintaining strong scaling behavior, using the supercomputer Fugaku. This algorithm enriches the body of classical high-performance methods and challenges current quantum advantage efforts.

Superconducting nanowire single-photon detectors with high count rates are important for quantum information processing, optical communication, and photon-starved imaging. Their timing performance is limited by excess jitter, though, and the underlying physics is not fully understood. This study identifies stochastic baseline fluctuations caused by the finite memory of ac-coupled readout circuits as an important source of timing jitter at high count rates, and establishes a quantitative framework to predict their impact. Also, under pulsed illumination the timing jitter is found to reach a maximum at about half of the laser’s repetition rate.

Volatile resistive switching in correlated-electron systems is promising for electronics applications, but the underlying physics remains obscured. Most studies have focused on inorganic thin films on substrates with strong thermal coupling to their surroundings, but here the authors investigate in a bulk organic single crystal with an exceptionally sharp metal-insulator transition. Bulk-sensitive microscopic NMR reveals the coexistence of metallic and insulating regions in the resistive-switched state, while weak thermal coupling to the surroundings allows temperature locking near the transition temperature and an “inverse Ohm’s law”, with voltage inversely proportional to current.

This Letter investigates the coupling mechanism of surface acoustic waves (SAWs) with spin waves (SWs) via micromagnetic analysis. The SAW magnetoacoustic excitation field is fully implemented (all strain and lattice-rotation terms included) in a realistic Co-Fe-B film with weak in-plane uniaxial anisotropy. Resonance alone does not guarantee efficient coupling; weak in-plane anisotropy can reshape the SAW-SW coupling, while lattice rotation can enhance and restructure the absorption features. Particular emphasis is put on the case where a SAW propagates parallel to the external magnetic field, a configuration of special interest for magnonics.

Quantum dot–based spin qubits require ultrafast, high-fidelity charge readout for quantum error correction and real-time feedback. Improving readout sensitivity has often required complex high-impedance resonators or specialized circuits. This work shows that optimizing a gate lever arm directly coupled to an in situ superconducting microwave resonator dramatically enhances readout sensitivity, achieving integration times at the tens of nanoseconds scale without the use of high-impedance devices, and revealing how readout noise evolves across distinct physical regimes. These results show a practical route toward faster, more scalable architectures for fault-tolerant quantum computing.

This Letter presents an innovative type of metamaterial that exhibits photonic-crystal-like Floquet scattering when exposed to an obliquely incident, weak electromagnetic probe wave. A laser-driven two-dimensional quantum well modulates the photoexcited electron density at the beat frequency of the laser radiation, behaving as a polychromatic source of radiation. Backward-propagating and surface-bound down-shifted satellites, including TE surface modes, are predicted. The proposed scheme provides a roadmap for extending photonic time crystals to the terahertz and near-infrared frequency domains.

Spin-squeezed states can surpass the standard quantum limit, yet their advantage is confined to a narrow phase range, complicating their use in noisy sensors. The authors present an adaptive Bayesian quantum estimation protocol that locks interferometry to its optimal operating point and incorporates phase noise into a reshaped likelihood function. Applied to quantum gravimeters and atomic clocks, this approach substantially enhances precision and outperforms conventional fringe-fitting protocols under noise. This framework establishes a noise-resilient pathway for entanglement-enhanced sensing, advancing high-precision measurements in geophysics, navigation, and timekeeping.

$XY$ (planar-spin) Hamiltonians arise in phase synchronization and retrieval and analog formulations of hard optimization, which motivates fast, low-power physical solvers. However, gain-based $XY$ systems that encode each spin with a single complex field can become trapped in metastable states. The authors introduce an annealer that represents each spin with two coupled complex components, and uses a graph-independent locking term that exploits the extra degree of freedom to bypass barriers. For challenging graph families, this higher-dimensional annealing improves ground-state recovery compared to one-component approaches, supporting more reliable photonic and analog $XY$ optimization.

Materials that remain ventilating while providing airborne sound insulation are highly desirable for everyday noise controllers. This review discusses lattice metamaterials as an innovative class of advanced structures capable of fulfilling both functions. The authors survey sound-insulation performance and mechanisms, and identify the most effective types of architectures among distinct categories of lattice metamaterials. In addition, they propose numerical strategies to enable accelerated design exploration.